Holography that obtains an information concerning to a sound source from a hologram of sound pressure emitted from the sound source and measured on a reference surface in the sound field of the sound source. It has been broadly uesed in civilian and munition industries. Informations to be obtained from the hologram may comprise a farfield directivity information, a nearfield vector intensity information, a surface velocity information, a total sound power information and so forth.
More particularly, the holography may be applied in an apparatus for finding out the enemy's soldiers in the munition industry. In a civilian industry, for example, it is applied to detect a sound source and then to eliminate the sound source or to build a soundproofing wall. Nowadays, growing demand to life environmental protection results in increasing a need for appropriately coping with any bothersome noise.
Studies of holography which are remarkably related to this invention are as follows: "Nearfield acoustic holography (NAH)-1. Theory of generalized holography and the development of NAH by J. D. Maynard, E. G. Williams and Y. Lee, disclosed in Journal of the Acoustical Society of America, Vol. 74, No. 4, pp1395-1413; "Nearfield acoustic holography (NAH)-2". Holographic reconstruction algorithms and computer implementation" by W. A. Veronesi and J. D. Maynard, disclosed in Journal of the Acoustical Society of America, Vol. 81, No. 5, pp1307-1322; U.S. Pat. No. 4,415,996 entitled "Nonwavelength-limited holographic sound field reconstruction" by J. D. Maynard and E. G. Williams; "Method of Spatial Transformation of Sound Fields--a unique technique for scan-based near-field acoustic holography without restrictions on coherence" by J. Hald, disclosed in Technical Review No 1, 1989, B&K publication; and "Broadband acoustic holography reconstruction from acoustic intensity measurement" by Loyau, J. C. Pascal and P. Gaillard, disclosed in Journal of the Acoustical Society of America, Vol. 84, No. 5, pp1744-1750.
In holography, a hologram is obtained on a reference surface that is so called "a hologram surface" and then the hologram is analized to estimate an acoustic property on any point in the environmental space. The hologram surface may be a plane or a cylindrical surface. In shape of the hologram surface, it is discriminated into a planar acoustic holography, a cylindrical acoustic holography and spherical acoustic holography.
In the planar acoustic holography, it is needed theoretically to measure sound pressure on an infite number of points in order to obtain a hologram showing distribution of the sound pressure on an infinitely great plane. In practice, however, a hologram is obtained by measuring sound pressure on a limited number of points considering time and cost taken in measurement.
Accuracy of the hologram is affected by density of measuring points, i.e., distance between adjacent measuring points. Accuracy of the hologram is inversely proportionated to the distance. Low accuracy of the hologram results in deteriorating accuracy of an estimated sound pressure in the sound field. Therefore, it is needed to provide technics for obtaining a highly accurate hologram while decreasing time and cost taken in measurement.
In U.S. Pat. No. 4,415,996 by W. A. Veronesi and J. D. Maynard, based on the thesis of "Nearfield acoustic holography (NAH)-2. Holographic reconstruction algorithms and computer implementation" disclosed in Journal of the Acoustical Society of America, Vol. 81, No. 5, pp1307-1322, "Nonwavelength-limited holographic sound field reconstruction" is described, in which a hologram is obtained by a microphone array comprising microphones on matrix positions of 16 rows by 16 lines. In this method, very highly cost has to be taken, because of the number of microphones up to 256.
In the thesises of "Method of Spatial Transformation of Sound Fields--a unique technique for scan-based near-field acoustic holography without restrictions on coherence" by J. Hald, disclosed in Technical Review No 1, 1989, B&K publication, and "Broadband acoustic holography reconstruction from acoustic intensity measurement" by Loyau, J. C. Pascal and P. Gaillard, disclosed in Journal of the Acoustical Society of America, Vol. 84, No. 5, pp1744-1750, there are described methods for measuring sound pressure in turn on plural points by a set of microphones fewer than the number of measurement positions. One of problems in these methods is that in order to measure sound pressure on each measurement position, a microphone has to be stayed on the position for a given time.
Aforementioned measuring methods have a basic limitation that accurate measurement of sound pressure is able to only the time when a microphone is stayed on a measurement position since there is no consideration about any relative motion between the microphone and the sound source. That is, it is not able to measure for a movable sound source. In measurement for a static sound source, scanning method in which sound pressure is measured during movement of the microphone.